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D2850-24 – Standard Test Method Technical Guide
📐 Specimen Types and Test Scope
ASTM D2850-24 defines the Standard Test Method for Unconsolidated-Undrained Triaxial Compression Test on Cohesive Soils. This test method is designed for cylindrical specimens of cohesive soils in an intact, compacted, or remolded state. Specimens are subjected to a confining fluid pressure in a triaxial chamber. A defining requirement is that no drainage of the specimen is permitted during the application of the confining fluid pressure or during the compression phase of the test.
The method provides data for determining undrained strength properties and stress-strain relations based on the measurement of total stresses only—stresses are not corrected for pore-water pressure. For unconfined conditions, Test Method D2166/D2166M applies. For consolidated-undrained strength with pore pressure measurement, Test Method D4767 is applicable.
| 🟦 Test Parameter | 📐 D2850 (UU) | ⚡ D4767 (CU) |
|---|---|---|
| 📏 Drainage During Confinement | 🚫 No | ✅ Yes |
| 🎯 Pore Water Pressure Measured | ❌ No | 🔬 Yes |
| 💪 Stress Type Reported | Total Stress | Effective Stress |
| 📊 Key Application | Total Stress Analysis | Effective Stress Analysis |
⚙️ Test Procedure and Speed Selection
The testing procedure mandates that the specimen is axially loaded at a constant rate of axial deformation (strain-controlled). This ensures a standardized loading condition. All observed and calculated values must conform to the guidelines for significant digits and rounding established in Practice D6026. The standard recognizes that the industry standard for data collection and calculation is provided, but the specific significant digits reported may be adjusted based on the user’s objectives and material variation.
⚠️ Strict No-Drainage Condition: To maintain the validity of the UU test, the system must be completely sealed. Drainage from the specimen is strictly prohibited during both the application of confining pressure and the axial compression phase. Any leakage compromises the undrained condition.
📊 Key Measured Properties and Reporting Units
The primary data produced by this test is the undrained shear strength (commonly denoted as Su) and the complete stress-strain relationship of the soil. Regarding units, the standard clearly states that SI units are regarded as the standard. Values given in parentheses are mathematical conversions to inch-pound units provided for information only. The inch-pound conversions utilize the gravitational system where the pound (lbf) represents a unit of force.
💡 Tip for Sampling and Preparation: For intact specimens, following Practice D1587/D1587M for Thin-Walled Tube Sampling is essential to minimize disturbance. Remolded or compacted specimens must be prepared to replicate the target density and moisture content accurately before placement in the triaxial chamber.
| 📏 Standard Number | 🎯 Title / Purpose |
|---|---|
| D653 | Terminology Relating to Soil, Rock, and Contained Fluids |
| D854 | Test Methods for Specific Gravity of Soil Solids by Water Displacement |
| D1587/D1587M | Practice for Thin-Walled Tube Sampling of Fine-Grained Soils |
| D2166/D2166M | Test Method for Unconfined Compressive Strength of Cohesive Soils |
| D6026 | Practice for Using Significant Digits in Geotechnical Data |
❓ Frequently Asked Questions
🔍 What are the main differences between D2850 (UU) and D4767 (CU)?
The key difference is drainage and stress type. D2850 prohibits drainage entirely and reports total stresses. D4767 allows drainage during confinement and measures pore-water pressure to report effective stresses.
💡 Can this method be used for sands or gravels?
No. Section 1.1 of the standard explicitly limits its scope to cohesive soils (intact, compacted, or remolded). Non-cohesive soils require different confinement and saturation procedures typically covered in other standards.
⚡ What units are mandatory for reporting results?
SI units are regarded as the standard. Inch-pound units in the standard are mathematical conversions for information only. Reporting results in non-SI units does not constitute nonconformance with this test method per Section 1.4.
📌 How does D2850 relate to the Unconfined Compression Test (D2166)?
While both measure undrained strength, D2166/D2166M specifically measures unconfined compressive strength (no lateral confinement). D2850 applies a confining fluid pressure to simulate the effect of in-situ overburden stress on cohesive soils.